Method of accommodating in volume expansion during solder reflow

ABSTRACT

Solder balls such as, low melt C4 solder balls, undergo volume expansion during reflow, such as may occur during attachment of chip modules to a PCB. Where the solder balls are encapsulated, expansion pressure can cause damage to device integrity. A volume expansion region in the semiconductor chip substrate beneath each of the solder balls accommodated this volume expansion. Air-cushioned diaphgrams, deformable materials and non-wettable surfaces may be used to permit return of the solder during cooling to its original site. A porous medium with voids sufficient to accommodate expansion may also be used.

This application is a divisional application of application Ser. No.09/845,448, now U.S. Pat. No. 6,686,664, filed Apr. 30, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and structures for attaching asemiconductor chip or chip carrier to a substrate and, moreparticularly, to methods and structures for attaching a semiconductorchip or chip carrier to a substrate using solder ball technology.

2. Background and Related Art

In the fabrication of electronic devices as, for example, during ballattach or card attach, low melt C4 (controlled collapsed chipconnection) solder balls on a chip carrier will reach their meltingtemperature and become liquid. Typically, for solder with a high tincontent, the volume expansion associated with this phase change canrange between 3 and 6%. If the C4 solder balls have been encapsulatedprior to this volume change, as is typically the case, the volumeexpansion is constrained and the resulting pressure may result in thesqueezing of this expanding volume of liquid into voids present in thesurrounding underfill and its associated interfaces. This volumeexpansion of solder may also result in opening any weak interfaces, likeunderfill to chip passivation (for example polyimide) or underfill tosolder mask interfaces. It is clear that the effect of such action couldresult in device failure.

SUMMARY OF THE INVENTION

In accordance with the present invention, structures are provided on thechip carrier to relieve pressure created by volume expanding solder uponheating and reflow. The structures are formed directly beneath thesolder balls or bumps. The pressure relief structure may be in the formof microchannels or vias, an air cushioned diaphragm, or porous orcompressible medium, like foam. The various structures act in a mannerto accept or accommodate the expanding or excess volume of soldercreated during melting to thereby minimize or avoid the creation ofpressure that may affect the region adjoining or surrounding the solderballs and the various material interfaces.

Accordingly, it is an object of the present invention to provideimproved methods of making connections in electronic devices, to enhanceoverall reliability of the product.

It is another object of the present invention to provide structureswhich act to accommodate expanding solder when it changed to the liquidphase.

It is yet another object of the present invention to provide a method ofattaching enclosed solder balls to connection pads by providingstructures that accommodate expanding solder upon reflow.

It is a further object of the present invention to provide structuresthat relieve internal pressures in an enclosed electronic packagingenvironment caused by the expansion of solder when going from the solidto liquid phase.

It is yet a further object of the present invention to provide methodsand structures that relieve pressure from solder reflow to therebyprevent damage to material interfaces in electronic devices.

These foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings, wherein like reference members representlike parts of the invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a cross-section of a typical Prior Art arrangement whereina semiconductor chip is positioned for electrical connection to asubstrate through an array of solder balls.

FIG. 2A shows an enlarged section of the arrangement shown in FIG. 1with one form of structure used to release pressure on reflow of solderballs.

FIG. 2B shows an enlarged section of the arrangement shown in FIG. 1with a further structure used to release pressure on reflow of solderballs.

FIG. 3 shows another enlarged section of the arrangement shown in FIG. 1with an air-cushioned form of structure used to relieve pressure onreflow of solder balls.

FIG. 4 shows yet another enlarged section of the arrangement shown inFIG. 1 with another air-cushioned form of structure used to relievepressure on reflow of solder balls.

FIG. 5 shows still yet another enlarged section of the arrangement shownin FIG. 1 with a compressible form of structure used to relieve pressureon reflow of solder balls.

FIG. 6 shows a further enlarged section of the arrangement shown in FIG.1 with a further porous form of structure used to relieve pressure onreflow of solder balls.

DETAILED DESCRIPTION

With reference to FIG. 1, there is shown a conventional arrangement ofsemiconductor chip and substrate. Substrate 3 may be a PCB type ofsubstrate or a ceramic substrate, for example. Substrate 3 may also be asingle chip module or a multi chip module (MCM) which is, in turn,attached to a substrate, such as a PCB. Chip 1 is shown positioned onsubstrate 3 with C4 solder balls or bumps 5, for example, positionedtherebetween. Solder balls 5 may, in fact, not be ball shape but may beshaped like bumps or be, very generally, globular in shape. FIG. 1 showsthe balls 5 somewhat elongated in shape but slightly truncated at theirends by conductive pads 7 and 9. Thus, the terms “solder balls” or“solder bumps” should not be taken to be limiting in shape but taken tobe more as a mass of solder. In this regard, it is clear that connectionis not necessarily limited to a C4-type or a flip chip solder connectionbut may, for example, be a BGA solder interconnect. Typically, solderballs 5 are first attached to conductive pads 7 on substrate 3. Pads 7may, for example, be copper pads. Chip 1 is then aligned so that itscopper pads 9, or other bump limiting metallurgy (BLM) structures, alignwith solder balls 5.

As further shown in FIG. 1, a layer of insulating material 11 surroundsand encapsulates solder balls 5. Typically, the chip and substrate padsare aligned to solder balls 5 and then the arrangement heated to reflowthe solder to make the connection. After connection is made, anunderfill is then dispensed between chip and substrate to provideencapsulation of the solder connections and support therefor.

Whatever technique is used to make connections and encapsulate same, itis clear that when encapsulated there is little room for expansion ofthe solder balls or connections on subsequent single or multiple reflow.Subsequent reflow may occur, for example, when there is subsequentattachment to a PCB, where substrate 3 is a single or MCM, or subsequentattachment to a card. It can also occur during preconditioning. Thisproblem is particularly severe for low melt single alloy solders.Typically, the volume expansion associated with high tin content soldersin going to the liquid phase is 3 to 6%. However, the problem may existfor any of a variety of solder alloys that exhibit high volume expansion(e.g. >3%) on melting and that will encounter additional reflow (melt)temperatures during assembly or preconditioning of the package.

With such volume expansion in an encapsulated environment, the phasechange instantaneously produces pressure that may result in thesqueezing of the excess volume into voids present in the surroundingunderfill or spacer, or produce a hydraulic force acting on thesemiconductor chip thus opening or delaminating any weak interfaces,such as, the underfill-polyimide and underfill-solder mask interfaces.In addition, solder bridging, solder migration to interfaces and solderdepletion within joints may occur. In this regard, it should beunderstood that the problems caused by solder volume expansion on reflowalso exist with second and subsequent levels of solder interconnects,such as, BGA solder joints that have been underfilled or encapsulated.Accordingly, the teachings of the present invention to solve suchproblems are equally applicable to second and subsequent levels ofpackaging. The teachings help in mitigating the above related problemsand provides for improving reliability of the electronic product.

In accordance with the present invention, several structuralarrangements are provided to relieve pressure created by volumeexpansion of solder during reflow. FIG. 2A is enlarged partial sectionshowing one of the solder balls of FIG. 1 with such partial sectionshowing one such structural arrangement for relieving pressure duringreflow. Microchannel, cavity or via 13 is shown beneath solder ball 5 toaccommodate expanding solder volume during reflow. Connection to othercircuitry here is through top surface metallurgy connected to pad 7. Inthis regard, each of the solder balls in the solder ball array isprovided its own independent microchannel or via to facilitateexpansion. These microchannels or vias may be, for example, laserdrilled by laser ablation through pads 7 (forming hole 8) and into thesubstrate 3 prior to mounting solder balls and chip to the substrate.

Representative dimensions for a 5% volume expansion of C4 solder ballsmight be A=140 μm, B=100 μm, C=45 μm and D=25 μm. Such dimensions wouldtypically approximate the maximum volume of the microchannel that isneeded to accommodate 5% volume expansion of solder. It should beunderstood, however, that, in general, the microchannel volume need notnecessarily be large enough to accommodate the total volume expansion ofthe solder but rather the microchannel volume may be optimized to belarge enough to sufficiently relieve pressure and limit stress build-upso that it is below the interfacial adhesion strength of the underfill.This, in turn, will depend on the type of underfill and passivation onthe die and the choice of solder mask material on the laminate.

Microchannel or via 13, in FIG. 2A, has a non-wettable surface 15 suchthat during reflow, the excess volume of solder would be forced intomicrochannel 13 thus relieving the pressure by accommodating the excessvolume without affecting the adjoining regions. Then, during cooling thesurface tension of the solder would force the solder back up onto copperpad 7 thus regaining its original ball-like shape. It should beunderstood that the Figures are not to scale and are only generallyillustrative of the shapes and sizes and are merely used to facilitate adescription and understanding of the invention.

FIG. 2B shows a pressure relief structure similar that shown in FIG. 2Abut rather than employ a single microchannel or via, multiplemicrochannels are employed under each solder ball, such as shown at 14and 16. As in FIG. 2A, holes in pad 7 may be laser ablated and then themicrochannels or vias 14 and 16 either ablated or etched into substrate3. Similar to FIG. 2A, the surfaces of microchannels or vias 14 and 16may be non-wettable.

Employment of multiple microchannels or vias, as shown in FIG. 2B, wouldbe particularly useful for BGA solder joints, such as, those employed inMCM-L (multi chip module-laminate) and CSP (chip size package)applications that have large contact surface areas. By using multiplemicrochannels, the microchannel depths may be reduced to achieve thesame total volume. Shorter microchannel depths have the advantage ofshorter return paths for solder upon solidification. A particularlyadvantageous shape for the microchannels would be conical, as shown inFIG. 2B, with E>D for each hole. Although two microchannels or vias 14and 16 are shown in FIG. 2B, it is clear that more than two holes couldbe employed. Typically, anywhere from 2 to 6 somewhat evenly spacedholes through pad 7 would work well although the number will be somewhatdependent upon the area of the pad surface. It should also be noted,that the single hole 13 in FIG. 2A could also be conical in shape withthe larger opening running through pads 7, similar to FIG. 2B.

FIG. 3 shows another structural arrangement for accommodating soldervolume expansion during reflow. In FIG. 3, via or cavity 17 is platedwith a layer 19 of conductive material, such as, copper. The plated via17, shown in contact with pad 7, is used to make connection to othercircuitry. Electrical connection can also be made directly to pad 7 fromthe surface. In this structural arrangement, pad 7 also acts as anair-cushioned diaphragm which functions to accommodate expanding volumeof solder into via 17 during reflow. In this regard, pad 7 issufficiently thin and elastic so as to flex without rupture in responseto the expanding volume of solder during reflow and, then, upon coolingreturn to its original state, as shown.

FIG. 4 shows a further air-cushioned diaphragm arrangement foraccommodating excess volume of solder during reflow. In thisarrangement, a flexible insulating layer 21, such as polyimide, is usedas a diaphragm over cavity 23. A hole or via 25 formed in pad 7 exposessolder ball 5 to layer 21. During reflow of solder ball 5, excess volumeof solder acts to depress layer 21 downwardly into cavity 23 toaccommodate the expanding volume. During cooling, the volume expandedinto the cavity via layer 21 is contracted and the air-cushioneddiaphragm returns to its original state, as shown.

FIG. 5 shows yet another structural arrangement for accommodating soldervolume expansion during reflow. In FIG. 5, a somewhat porous, deformablelayer 27 is exposed to solder ball 5 by way of a hole or aperture 29.Layer 27 has a top surface that is closed and continuous (non-permeableto solder) and compliant. Upon application of heat to reflow solder ball5, excess solder caused by volume expansion during the liquid phase isforced downwardly through hole 29 causing deformable layer 27 tocompress to relieve the resultant pressure. The liquid solder on reflowdoes not enter into the pores or voids of layer 27 since its top surfaceis non-permeable. Since compression is local to each cell, each cell isclosed off from the others. In addition to having the top surface oflayer 27 non-permeable, a thin, flexible, non-permeable membrane mayalso be formed on its surface. Upon cooling, the liquid solder is drawnback up through hole 29 onto pad 7 to its original position, as shown.This is a result of both surface tension and pressure from thedeformable layer. Typical materials that may be used for layer 27 areRO2800 Rogers material with a non-permeable membrane, like polyimide,adhered to the top surface such that it acts as a closed-cell material.Cellular silicone can also be converted to a closed-cell structurethrough adhesion of polyimide to its surface. Thicknesses for layer 27may range from 75 μm to 100 μm.

FIG. 6 shows yet a further structural arrangement for accommodatingsolder volume expansion during reflow. In FIG. 6, a porous, rigid layer31 is employed, in contrast to the deformable layer 27 in FIG. 5. In thestructural arrangement of FIG. 6, when solder ball 5 is subjected toheat to reflow the solder, the volume expansion of the solder in theliquid phase is accommodated by being absorbed into the pores or voidsof layer 31. In this regard, the surface of layer 31 is open, i.e., thevoids are accessible at the surface portion of the layer exposed to hole29. Thus, the voids in regard to layer 31 act as pressure reliefreservoirs. Layer 31 may be made, for example, of porous ceramicmaterial with non-wettable voids. Again, upon cooling the liquid solderis drawn up through hole 29 to reform on pad 7, as shown.

To ensure that the porous area under solder ball 5 is isolated from theporous areas under adjacent solder balls, isolation trench or region 33may be formed. Isolation region 33 may be made by forming a trench inrigid layer 31 around the region beneath solder ball 5. The trench maythen be backfilled with an isolating material, such as, polyimide or anoxide. The trench may be etched or laser profiled through layer 31 tosubstrate 3. Isolation region 33 prevents unwanted migration of thesolder, absorbed during reflow, from interacting with the solderabsorbed during reflow of an adjacent site. Rigid layer 31 may be madeof a conventional ceramic material fabricated to exhibit voids. Layer 31may be 75 μm to 100 μm thick.

Rather than form isolation region 33 in the porous rigid layer 31, thesubstrate, itself, may be used to form an isolation region. This may beachieved by masking a region of substrate 3 around the site of thesolder ball that is to act as the isolation region, and then etchingback the substrate inside the region. Thereafter the etched region isbackfilled with the porous, rigid material.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred embodiment of thepresent invention without departing from its true spirit. It is intendedthat this description is for purposes of illustration only and shouldnot be construed in a limiting sense. The scope of this invention shouldbe limited only by the language of the following claims.

1. In a method of fabricating electronic devices wherein reflow ofencapsulated solder occurs, comprising: forming a volume expansionregion adjacent to said encapsulated solder having volume dimensionssufficient to accommodate enough of the volume expansion of saidencapsulated solder during reflow so as to prevent damage, said volumeexpansion region including an air-cushioned diaphragm that flexes duringvolume expansion of said solder; and applying heat to said encapsulatedsolder to cause solder reflow to the liquid phase and expand into saidvolume expansion region and wherein during cooling the said solder thatexpanded into said volume expansion region is drawn-back to its originalencapsulated site.